The duration of hepatitis B vaccine-induced immunity was studied in a group of 54 seronegative health professionals who received plasma-derived hepatitis B vaccine (Merck's Heptavax) in 1978 and 1979. Five to seven years later, 52 vaccinees received a booster dose of yeast recombinant hepatitis B vaccine (Merck's Recombivax). Of 54 vaccinees, 47 (87 percent) had a favorable anti-HBs response (greater than 10 S/N RIA units) and 7 (13 percent) had low (2.1-10 S/N) or undetectable levels (less than 2.1 S/N) one year after primary immunization. After five to seven years, the anti-HBs values had declined to undetectable levels in 25 percent and to low levels in 23 percent. A booster dose of vaccine induced an anamnestic response in 90 percent of vaccinees by two weeks. The results of this study indicate that persons who respond favorably to primary immunization may be protected for at least seven years.